Breakdown of theZ=8Shell Closure in UnboundO12and its Mirror Symmetry

An excited state in the proton-rich unbound nucleus $^{12}\mathrm{O}$ was identified at 1.8(4) MeV via missing-mass spectroscopy with the $^{14}\mathrm{O}(p,t)$ reaction at $51\text{ }\text{ }A\mathrm{MeV}$. The spin-parity of the state was determined to be ${0}^{+}$ or ${2}^{+}$ by comparing the measured differential cross sections with distorted-wave calculations. The lowered location of the excited state in $^{12}\mathrm{O}$ indicates the breakdown of the major shell closure at $Z=8$ near the proton drip line. This demonstrates the persistence of mirror symmetry in the disappearance of the magic number 8 between $^{12}\mathrm{O}$ and its mirror partner $^{12}\mathrm{Be}$.